A characteristic of wave signals is that they travel through a particular medium radially from a wave source. For sound waves, which are a type of wave signal, for example, unidirectional microphones having maximum sensitivity in the forward direction and gun microphones having sharp directivity, have been developed as devices for detecting sound waves from a specific direction.
Well known methods in which a plurality of microphones are used include microphone array technologies and a method for creating directivity in a target direction by suppressing wave signals arriving other than from a target direction by detecting the direction of the wave source. When such approaches are adopted, unidirectivity can be established in any direction by arranging three or more microphones in such a way that they do not form a straight line.
If a plurality of microphones are used to detect the direction of a wave source, then it is necessary to calculate the time difference of the signals arriving at each microphone, and correlation calculation is generally used for this calculation. Alternatively, instead of correlation calculation, Japanese Patent Application No. 2002-078697 discloses a method in which few operations are used to calculate the time difference between wave signals that arrive at each microphone using difference signals and differential signals of the received signals.
However, a general problem with cardioid-type unidirectional microphones has been that although there is significant suppression of wave signals from the rear, there is little suppression of wave signals from the sides, and thus only broad directivity has been possible. On the other hand, although gun microphones have sharp directivity, the fact that a large sound tube must be provided in the direction of the wave source means that they require a larger set up space than general microphones and thus are not easily incorporated into compact devices. Similarly, microphone arrays require a large aperture in order to achieve sharp directivity, and thus require a larger set up space than general microphones, which has made it difficult to incorporate them into compact devices.
In addition, with the method using a plurality of microphones to detect the direction of a sound source and thereby create directivity, it is necessary to convert the analog input signals into digital signals at a high sampling rate and then perform correlation calculation, which requires a large number of operations for a large volume of sampled data. Accordingly, such methods are not easily adopted in real time applications and are also not easily achieved with processors having limited computing power.
Furthermore, incorporating microphones into compact devices results in a narrow aperture and very short delay times, and thus there is the problem that it is difficult to precisely calculate the delay time with methods using correlation calculation.
To solve the foregoing problems, it is an object of the present invention to provide a wave signal processing system and method with which processing is carried out using a small aperture and with which sharp directivity can be realized using few operations, as a method for detecting the direction of a wave source that is necessary when directivity is created using a plurality of microphones.